PROJECT SUMMARY Peripheral T-cell lymphomas including cutaneous T-cell lymphomas (CTCL) are poorly understood and many subtypes exhibit aggressive clinical course and high mortality with short (2-year) life expectancy after diagnosis. Poor understanding of mechanisms underlying the pathogenesis of CTCLs contributes to suboptimal diagnostic subcategorization and lack of targeted therapies. Mycosis fungoides (MF) and Sezary syndrome (SS) are two major forms of CTCL. Clinical staging and outcome of MF and SS is dependent on the quantity of neoplastic cells in the blood. While SS is an aggressive disease with poor overall survival (42.3% at 5 years and is by definition diagnosed at late stage disease, there are many patients with low tumor burden (Early-stage CTCL) who suffer from delay in diagnosis due to the subjective nature of the diagnostic criteria. The mean diagnostic delay (measured as the time from emergence of skin lesions to the diagnosis of SS) is long (4.2 year; median 2.8 years) with a significant variation (1 month to 32 years). Thus, there is a critical need for improved methods for early disease detection. Furthermore, the identification of genomic disease biomarkers that would establish early diagnosis and provide prognostic information would clearly have benefit for patients with this form of aggressive CTCL. Until recently, recurrent genetic alterations that can be exploited as diagnostic and prognostic biomarkers have not been described in these tumors. Our laboratory and other investigators recently described the genomic landscape of CTCL utilizing a comprehensive integrated strategy including whole genome sequencing, whole exome sequencing and array comparative gnomic hybridization. Highly recurrent gene mutations targeting epigenetic and chromatin remodeling and JAK-STAT and other pathways were identified in pre- treatment samples of CTCL. Based on these results, it is our central hypothesis that somatic alterations involved in the pathogenesis of CTCL in conjunction with high-throughput sequencing (HTS) of the T-cell receptor (TCR) can be utilized for early diagnosis and that genetic profiles and sensitive and quantitative detection of clonal T-cell populations can serve as prognostically-relevant biomarkers of CTCL. Accordingly, in Specific Aim 1, we propose to investigate the utility of a multivariate model based on targeted next generation sequencing (NGS) and HTS-TCR assays for the diagnosis of ES-CTCL and Late- stage CTCL (SS). In Specific Aim 2, we will investigate the impact of the genetic mutations and HTS-TCR on the prognosis of ES-CTCL and SS using a retrospective cohort. In Specific Aim 3, we will assess the performance of the multivariate model for assessing the prognosis of ES-CTCL and SS using a prospective cohort. The overall impact of this proposal is the development of targeted genomic assays that will lead to early and accurate diagnosis of CTCL and improve methods to assess early disease detection and prognosis.